1. Field of the Invention
The present invention relates to an optical pickup module with aberration correctability and an optical information reproduction apparatus using the pickup.
2. Description of Related Art
A background art in this technical field is disclosed, for example, in JP-A-11-110802. This Japanese bulletin contains, as an objective of the invention disclosed therein, the recitation which reads as follows: “ . . . provide an aberration correcting device capable of effectively correcting wave-surface aberration occurring due to the tilting of an optical axis while enabling miniaturization with simplified arrangement and an information reproducing apparatus having this aberration correction device.” It also discloses in the context of a solving means that “transparent electrodes 10c and 10d which are divided into pattern electrodes 30a, 30b, 31a, 31b, 32 and 40a, 40b, 41a, 41b, 42 each having a shape corresponding to a distribution of wavefront aberrations are formed on the both surfaces of a liquid crystal layer capable of giving to a light beam passing therethrough a phase difference depending upon the molecule direction thereof, thereby controlling the polarity and the value of a voltage being applied to each pattern electrode in a way corresponding to a tilt angle in either a detected tangential direction or a radial direction to thereby change the phase difference of the pass-through light beam in units of liquid crystal regions as partitioned by respective pattern electrodes in such a way as to cancel out the presently occurring wavefront aberration. At this time, the voltage applied is inverted in its polarity to permit application of the necessary potential difference to a liquid crystal element.”
A prior known example of the above-noted wavefront aberration correction device is designed so that transparent electrodes of prespecified shapes are disposed on the both surfaces of a liquid crystal (LC) element. In the aberration correction device using this LC element, each of the transparent electrodes which are disposed to interpose the LC element therebetween is driven to create a potential difference to thereby change the orientation of LC molecules and then locally vary the refractivity due to a difference in LC molecule orientation and thus give a local phase change to the light beam passing through this LC element for correction or “amendment” of the wavefront aberration.
Specifically, the above-cited JP-A-11-110802 discloses therein a configuration of an LC aberration correction device, which includes transparent electrodes 10a-10e each having a predefined shape on the surface of an LC element 1 as shown in FIG. 2—this diagram shows an exemplary electrode pattern of prior art coma aberration correction device—for the purpose of correcting coma aberration which is a wavefront aberration that appreciably affects the optical performance of an optical pickup.
Unfortunately, this aberration corrector device is faced with two major serious technical problems which follow.
An explanation will first be given of the first problem. In the prior art aberration corrector device, the light beam that is successfully correctable in its wavefront aberration is limited to a single kind. This can be said because the transparent electrodes disposed on the surface of LC element are usually designed in shape and size in such a way as to obtain the optimum aberration correction performance for the light beam having a predetermined effective beam diameter. Adversely this means that regarding a light beam having an effective beam diameter different from the effective beam diameter, its aberration correctability becomes extremely impaired even when performing aberration correction by an aberration correction device identical to the corrector device.
See Table 1 below, which shows a comparative example of the aberration correction factor per design condition in prior art coma aberration correction device.
TABLE 1AberrationAberrationCorrectionCorrectionFactor (%)Factor (%)for DVD Lightfor CD LightBeam DiameterBeam Diameterof 2.0 mmΦof 1.6 mmΦDesignCase A61.318.7ConditionsCase B11.361.3Case C28.730.7In the table above, the aberration correction factor, Fc, is defined as follows:Fc=(Ai−Ar)/Ai×100(%),where, Ai is the initial aberration amount in route mean square (rms) value, and Ar is the residual aberration after correction in rms value. In Case A of Table 1, system design was made to achieve the best possible aberration correction factor with respect to a digital versatile disc (DVD)-use light beam having its effective diameter of 2.0 mmΦ. In Case B, design was made to attain the best aberration correction factor relative to a compact disc (CD)-use light beam with an effective diameter of 1.6 mmΦ. In Case C, design was done to get the best aberration correction factor for a light beam having its effective diameter of an intermediate value between those of the DVD beam and the CD beam—e.g., 1.8 mmΦ.
For example, Table 1 above indicates an exemplary coma aberration performance difference occurring due to a difference in effective beam diameter. The aberration correction device as used herein is a prevailing coma aberration corrector device of the LC type having the transparent electrode pattern shown in FIG. 2 as taught from JP-A-11-110802. Additionally the aberration correction factor as indicated in Table 1 is the ratio of an amount of coma aberration removed by the aberration corrector to the prespecified initial (prior to correction) coma aberration, which is an effective parameter for evaluation of the aberration correction performance.
In Case A of Table 1 an LC aberration correction device is used which is under optimum design of transparent electrode shapes and sizes in such a way as to maximize the aberration correction factor relative to a DVD read light beam having an effective beam diameter of about 2.0 mmΦ and a wavelength of 658 nm. As apparent from this table, in this case, the aberration correction factor of more than 60% is attainable for the DVD-use light beam. However, when coma aberration correction is carried out by the same aberration corrector device for a CD read light beam having its effective beam diameter of about 1.6 mmΦ and wavelength of 785 nm, the resulting aberration correction factor is as low as about 11%.
Adversely, as shown in Case B in Table 1, the use of an LC aberration corrector device with the transparent electrode shapes and sizes being optimally designed to permit the aberration correction factor to become maximal (about 60% or more) relative to the CD-use light beam would also result in achievement of a mere aberration correction factor of about 11% for the DVD light beam in this case.
When optimally designing the transparent electrode shapes and sizes of the aberration corrector device to ensure that the aberration correction factor becomes maximized relative to a light beam having its effective beam diameter of 1.8 mmΦ, which is an intermediate value between that of the DVD light beam (2.0 mmΦ) and the CD light beam (1.6 mmΦ) as shown in Case C of Table 1, the resultant aberration correction factor stays as low as about 30% for both of the DVD and CD light beams.
It is thus apparent that in the above-noted prior art aberration correction device, even when performing the optimum design of transparent electrodes in any possible way, it is impossible to achieve the optimum solution capable of obtaining excellent aberration correction performances for both of the two light beams that are different in effective beam diameter from each other.
In recent years, in order to enable either a single optical pickup or an optical information reproduction apparatus to perform playback of a plurality of types of optical discs, optical pickup modules become into wide use, which are arranged to permit multiple kinds of light beams different in wavelength and effective beam diameter from one another to travel in substantially the same optical path. In this type of optical pickups, it is evidently advantageous, in viewpoints of the size of the optical pickup per se and the number of components and the cost, to provide the capability of successfully correcting all possible wavefront aberrations of every light beam in the same aberration correction device, rather than an approach to disposing separate aberration corrector devices in units of respective light beams. However, as stated previously, the prior art fails to disclose the above-noted problems and any teachings as to the configuration capable of successfully correcting together wavefront aberrations of multiple kinds of light beams different in effective beam diameter from one another by use of a single aberration correction device.
The second technical problem will next be discussed. In addition to the above-noted first problem, the prior art aberration correction device suffers from a problem which follows: the creation of a relative position deviation or displacement between this aberration corrector device and a light beam falling thereonto would result in an appreciable decrease in aberration correcting performance.
FIG. 3 is a graph showing, in case a prior art aberration correction device having the known standard transparent electrodes shown in FIG. 2 as one example showing the above-noted problems, a plot of the relationship of a relative displacement amount upon occurrence of the above-stated relative displacement between this aberration corrector device and its incident light beam versus an aberration correction factor in such event. Note that the results shown herein are the calculation results obtained in case fixation is done in a state that the best possible aberration correction performance is obtainable when the potential difference being applied to each electrode, i.e., phase difference, is such that a relative displacement is zeroed.
As apparent from this graph, when the relative displacement is zero, that is, when the incoming light beam falls onto the aberration corrector device without exhibiting any displacement, the best aberration correction performance (with the aberration correction factor of 60% or more) is obtainable, although the aberration correction performance rapidly drops down with an increase in relative displacement: at the relative displacement of 0.2 mm, the aberration correction factor decreases to almost 0%.
In this way, with the prior art aberration correction device, its aberration correction performance can noticeably decrease even upon occurrence of a tiny relative displacement. However, in the actual optical pickup, it is unavoidable in any way that relative displacement of about several tens of μm occurs between the light beam and the aberration corrector device due to the presence of attachment position variations occurring during assembly of such pickup. Accordingly, whether superior aberration correction performance is achievable even in the presence of such relative displacement is an important issue which affects the optical pickup's performances.
However, the prior art discloses neither the above-noted problems nor effective means for avoiding the reduction of aberration correction performance occurring due to relative displacement.